Sample-and-hold circuits are used for sampling values of continuous time-varying amplitude signals at discrete time intervals. The sampling may, for example, be done as part of an analog-to-digital signal conversion process, wherein amplitude values of an analog input signal are sampled at regular clocked time intervals for conversion into binary digital values, to give a digital output representation of the time-varying input. Digital signal processing may then be readily applied to the converted signal before converting the processed signal back into analog form for control or other purposes. In a typical application, the rate of sampling must at least be equal to twice the highest relevant frequency component (viz. bandwidth) of the original signal (the Nyquist frequency) in order to accurately capture the meaningful information.
A conventional sample-and-hold circuit 10 is shown schematically in FIG. 1. A continuous, time-varying voltage signal Vin applied at input terminal 12 is connected through a switch 14 to charge a hold capacitor 15 (Ch), one terminal of which is connected to an output buffer 16 and the other terminal of which is connected to ground. Output buffer 16 may, for example, be a unity gain amplifier. Switch 14 is controlled by a sample-and-hold timing (or clocking) signal applied at node 17, so that when switch 14 is switched "on," the voltage at capacitor node 19 (and hence the output voltage Vout at output node 21) tracks the voltage applied at input node 12, and when switch 14 is switched "off," the last voltage applied at input node 12 is "held" at node 19 (and thus at node 21) by the charge on capacitor 15.
Switch 14 is often implemented as a MOS (metal-oxide semiconductor) transistor. When switch 14 is implemented as an NMOS transistor, for example, the input voltage Vin is passed through the N channel when a high gate voltage (e.g., Vdd) is applied at node 17 ("phase A") and is blocked when a low gate voltage (e.g., ground) is applied at node 17 ("phase B"). This causes Vout to track the varying magnitude of Vin during phase A (i.e., Vout=Vin), but "hold" the last phase A value of Vin during phase B (i.e., Vout=Vin (t.sub.i)). There are, however, two types of capacitance inherent in the MOS transistor that contribute along with the pass-through input voltage Vin to the captured charge at node 19 when the transistor is turned "off." One is the gate-oxide capacitance (Cox). The other is the gate-drain parasitic capacitance (Cgd). Capacitance Cox is the capacitance caused by locating the metal and dielectric oxide directly over the channel. When the MOS transistor is "on," in order for current to flow, charge is present at the surface of the channel, under the oxide. When the same transistor is then turned "off," that charge must be dissipated, some going to the drain and some going to the source. This affects the charge stored on capacitor 15 (voltage "held" at node 19) when switch 14 is turned "off." The charge contribution of Cox is proportional to Vin.times.Cox, where Cox is the gate oxide capacitance of transistor 14. Capacitance Cgd is a parasitic capacitance due to physical overlap between the gate and drain terminals, as well as from any coupling between the gate and drain connections, in the MOS transistor layered structure. (Where source rather than drain is connected to node 19, the terms "drain" and "Cgd" should be understood to include "source" and "Cgs" in the context of this discussion and the described invention.) Capacitance Cgd exists whether the transistor is turned "on" or "off" and, thus, also contributes to the charge stored on capacitor 15 (voltage "held" at node 19) when switch 14 is turned "off." The charge contribution of Cgd is proportional to Vin.times.Cgd, where Cgd is the gate-to-drain parasitic capacitance of transistor 14.
Because the input signal is voltage magnitude time-variant, the influence on the "held" voltage of sample-and-hold circuit 10 due to Cox and Cgd is a function of the input voltage. Capacitance Cox depends on the difference between the applied gate voltage and the voltage at node 19. For example, 3.0 volts may be applied to the gate to turn transistor 14 "on"; however, when transistor 14 is "on," the voltage at node 19 will track the (varying) voltage at input node 12. Thus, the amount of charge distributed due to Cox and Cgd at node 19 when the transistor is turned "off" will vary with the level of the input voltage. If the effect of Cox and Cgd on the captured value were constant each time the transistor were turned "off" to "hold" the sample, it could be readily taken into account. It would simply be a constant voltage "pedestal" that would raise or lower the signal by the same amount. But, because the amount of influence on the captured signal varies non-linearly as a function of the magnitude of the input signal, it causes a problem. With decreasing channel dimensions, the influence of Cox on the "held" value is lessening; however, the influence of Cgd is becoming more significant. There exists, therefore, a need to compensate for the adverse effects of Cox and Cgd in sample-and-hold circuits.